An organic light emission phenomenon is an example of converting electric current into visible rays through an internal process of a specific organic molecule. The principle of the organic light emission phenomenon is based on the following mechanism. When an organic material layer is disposed between an anode and a cathode, electrons and holes are injected from the cathode and the anode, respectively, into the organic material layer if voltage is applied between the two electrodes. The electrons and the holes which are injected into the organic material layer are recombined to form an exciton, and the exciton is reduced to a bottom state to emit light. The organic light emitting device using the principle may be generally constituted by a cathode, an anode, and an organic material layer that is interposed therebetween, for example, an organic material layer that includes a hole injection layer, a hole transporting layer, a light emitting layer and an electron transporting layer.
The materials used in the organic light emitting device are mostly pure organic materials or complexes of organic materials with metals, and may be classified into a hole injection material, a hole transporting material, a light emitting material, an electron transporting material, an electron injection material and the like, according to the use thereof. Here, an organic material having a p-type property, that is, an organic material which is easily oxidized and electrochemically stable when the material is oxidized, is usually used as the hole injection material or the hole transporting material. Meanwhile, an organic material having an n-type property, that is, an organic material which is easily reduced and electrochemically stable when the material is reduced, is usually used as the hole injection material or the hole transporting material. As the light emitting layer material, an organic material having both p-type and n-type properties is preferred, which is stable when the material is oxidized and when the material is reduced. When an exciton is formed, a material having high light emission efficiency for converting the exciton into light is preferred.
In addition to what is mentioned above, it is preferred that the material used in the organic light emitting device further has the following properties.
First, it is preferred that the material used in the organic light emitting device has excellent thermal stability. The reason is that joule heat is generated by movement of electric charges in the organic light emitting device. NPB, which has currently been used as the hole transporting layer material, has a glass transition temperature of 100° C. or lower, and thus it is difficult to apply to an organic light emitting device requiring a high electric current.
Second, in order to obtain an organic light emitting device that is capable of being driven at low voltage and has high efficiency, holes or electrons which are injected into the organic light emitting device need to be smoothly transported to a light emitting layer, and simultaneously the injected holes and electrons need to be prevented from being released out of the light emitting layer. For this purpose, a material used in the organic light emitting device needs to have an appropriate band gap and appropriate HOMO and LUMO energy levels. A LUMO energy level of PEDOT:PSS, which is currently used as a hole transporting material in an organic light emitting device prepared by a solution coating method, is lower than that of an organic material used as a light emitting layer material, and thus it is difficult to prepare an organic light emitting device having high efficiency and a long service life.
Moreover, the material used in the organic light emitting device needs to have excellent chemical stability, electric charge mobility, interfacial characteristic with an electrode or an adjacent layer, and the like. That is, the material used in the organic light emitting device needs to be minimally deformed by moisture or oxygen.
Furthermore, an appropriate hole or electron mobility needs to be assured so as to balance densities of the holes and of the electrons in the light emitting layer of the organic light emitting device to maximize the formation of excitons. Additionally, it needs to be able to have a good interface with an electrode including metal or metal oxides so as to assure stability of the device.
Accordingly, there is need for developing organic materials having the above-described requirements in the art.